That weapon can deal great damage to armor, but not with a slash. You'd use a chop.

Or Regular attack by Dark Souls terminology.

Well if you want to wax semantics. I was envisioning a weapon like a naginata, which the closest DkS equivalent would be the BK Halberd I think. Those kinds of weapons are designed for slashing motions, but they carry enough weight that if you banged one into a steel plate I'd think you'd do some damage. Especially coming from someone with training. I'd agree that smaller weapons designed for slashing probably wouldn't be all that effective, unless you were trained enough to be able to aim for eye slits or vulnerable joint spots with any kind of accuracy, but there are larger weapons that are designed to do similar things.